1. Field of the Invention
This invention relates to a process for polymerizing vinyl chloride alone or a mixture of vinyl chloride and other vinyl monomer(s) with one or more hydroxy containing monomers, and the vinyl chloride copolymers obtained thereby. More particularly, the invention relates to a suspension polymerization process for polymerizing vinyl chloride with a hydroxy containing monomer obtained by the partial allylation of dihydroxy or polyhydroxy alcohols.
2. Summary of Related Art
Vinyl chloride homopolymers have been widely employed in numerous industrial applications for many years. Not suprisingly, there have been numerous attempts to improve the physical properties of these polymers. For example, early attempts to modify the characteristics of polyvinyl chloride resins were made by Nelson, et al., as is disclosed in U.S. Pat. No. 3,706,722. Nelson et al. describe a process, known as a phase inversion process, for polymerizing polyvinyl chloride resin. Unlike a typical suspension polymerization, the monomer is in the continuous phase up to approximately 10% conversion. Thereafter, additional water is metered into the reactor to make the water be the continuous phase and the monomer the discontinuous phase.
This phase inversion process is the combination of mass polymerization in the early stage of polymerization and the subsequent inversion to suspension polymerization. The shortcomings of this technology are the complexity of the operation and the heavy polymer build-up on the internal surfaces of the polymerization equipment. Overall shapes of the polymer particles are not spherical, which makes the funnel flow time very long. In other words, the processing of the polymer takes a relatively long time. While the polyvinyl chloride resins obtained thereby show improvement in porosity, they do not have any functional groups permitting wide versatility in applications of the polymer.
U.S. Pat. No. 4,755,539 discloses an alternative method for modifying the surfaces of polyvinyl chloride resin particulates. The polymerization depends on the specific choice of an ion sensitive water-soluble dispersant. The charging procedure is described as follows:
(a) charge water and ion sensitive primary dispersant. PA1 (b) agitate the water and primary dispersant until an emulsion is formed. PA1 (c) stop the agitation. PA1 (d) charge the monomer(s). PA1 (e) charge a solution comprising a solvent and catalyst and optionally the secondary dispersant. PA1 (f) allow the time for the catalyst to be diffused through the monomer layer. PA1 (g) start the agitation. PA1 (h) conduct the polymerization until 1 or 2% conversion is reached. Then add NaOH to desorb the primary dispersant from the monomer droplet. PA1 (i) continue the polymerization to the point of the desired degree of polymerization. PA1 (a) polymerize a mixture of vinyl chloride and vinyl acetate in acetone. PA1 (b) separate and recover the resin from the resultant varnish as a completely dry, acetone free. PA1 (c) redissolve the resin in a mixture of methanol and methyl acetate. PA1 (d) hydrolyze the resin in the mixture. PA1 (e) separate and recover the resultant terpolymer of vinyl chloride, vinyl acetate, and vinyl alcohol as well as the mixture of solvents.
The shortcomings of this technology are the quite delicate operation to desorb the primary dispersant and the dependence on single choice of primary dispersant. This technique specifically excludes high hydrolysis polyvinyl alcohol greater than 70% and methyl cellulose as a primary dispersant because of the formation of skins on the surfaces of the resin particulates. The morphology of the polymer is non-spherical, which will restrict the areas of the product applications. Also, the resins do not have any functional groups, which will limit the versatility of the applications further.
Functional groups have been incorporated into polyvinyl chloride resins, typically to make good protective coating materials. Those functional groups improve abrasion resistance and impart flexibility to the resins. The coating materials are prepared by dissolving the polymers into ketone or ester solvents. Mainly, two functional groups are utilized. They are carboxyl and hydroxyl groups.
U.S. Pat. No. 2,147,154 discloses a method for polymerizing vinyl chloride with acrylic acid in emulsion. The products include 0.5 to 3% of acrylic acid.
One of the known methods of incorporating hydroxyl groups in polyvinyl chloride is through vinyl alcohol which is obtained from vinyl acetate with hydrolysis. Terpolymers of vinyl chloride, vinyl acetate, and vinyl alcohol are made by the steps:
This process several disadvantages: the high cost due to the double recovery of dry resin and solvent; the restricted amount of hydroxyl groups in the terpolymer because of the difficulty of making the copolymers having vinyl acetate content higher than 15%; the environmental concern and cost out of the usage of hydrocarbon solvents; the limitations in molecular weight as is typified by solution polymerizations; and the difficulty in obtaining controlled particle size and porosity via precipitation from solution.
U.S. Pat. No. 2,852,499 describes a method which reduced the cost disadvantage somewhat. This method uses solvent mixtures such a methoxyethanol, ethoxyethanol, methoxyethyl acetate, and dioxane. Both the polymerization of vinyl chloride with vinyl acetate and the subsequent hydrolysis were conducted in the same solvent. This process is still beset with the fundamental problems of hydrocarbon solvent usage, and separation and recovery of the resin as well as the solvent.
Vinyl chloride has also been copolymerized with allyl alcohol and dibutyl fumarate, as disclosed in U.S. Pat. No. 3,036,029. The mixture of allyl alcohol, vinyl chloride, and dibutyl fumarate was polymerized at 120.degree. F. in the absence of any solvent. The end product of this bulk or mass type polymerization was a clear colorless syrup comprising terpolymer dissolved in unreacted monomers. This process also faces the difficulty and cost of separation and recovery of monomers and the resin.
U.S. Pat. No. 3,257,360 illustrates a technique for copolymerizing vinyl chloride with vinyloxyethanol and fumaric ester. The monomer mixtures of vinyl chloride, vinyloxyethanol, and dibutyl fumarate were polymerized at 60.degree. C. with azobisisobutylnitrile without any solvent. The polymerization products were put into petroleum ether, where the vinyl chloride-vinyloxyethanol-dibutyl fumarate terpolymers precipitate immediately. This process also can not avoid the usage of hydrocarbon solvents, and it limits the polymerization to low molecular weight polymer.
Resins of polyvinyl chloride have also been made to contain hydroxyl groups by emulsion polymerization with hydroxyl alkyl acrylates. U.S. Pat. No. 2,686,172 illustrates the polymerization of vinyl chloride, alkyl acrylates, and hydroxy alkyl acrylates.
Recently, different approaches have been taken to modify polyvinyl chloride. Vinyl chloride copolymers have been used as a binder with a magnetic powder for a magnetic recording medium, as disclosed in U.S. Pat. No. 4,983,311 and U.S. Pat. No. 5,064,730.
U.S. Pat. No. 4,983,311 describes a vinyl chloride copolymer in which two functional groups, hydroxyl group and carboxylic acid group are simultaneously incorporated into the polymer binder via a homogeneous solution polymerization process. In Example 1 of this patent, the following recipe, all in parts by weight, was used:
______________________________________ batch charge vinyl chloride 100 glycerin monoallyl ether 3 acetone 180 benzoyl peroxide 2 metering maleic anhydride 3 vinylidene chloride 10 methanol 40 ______________________________________
Thus, this is not a typical suspension polymerization. Suspension polymerization makes use of a single solvent, water, as the continuous phase with monomer droplets suspended therein to form a non-homogeneous mixture. If other solvents are used, they are in minor portion and the mixture remains heterogeneous. In the invention described in U.S. Pat. No. 4,983,311, a mixture of solvents, such a acetone and methanol or water and methanol, is used in an almost 50/50 ratio. Also, the use of 2% catalyst with respect to monomers is unusually high, and can limit the polymerization to lower molecular weights. The usage level of catalyst in a suspension polymerization is generally 0.01 to 0.5 at most per 100 parts by weight of monomer(s). The dependence on the solvents poses environmental problems and increases manufacturing cost.
U.S. Pat. No. 5,064,730 discloses an approach quite similar to that in U.S. Pat. No. 4,983,311. A vinyl chloride copolymer containing hydroxyl groups as well as quaternary ammonium salt was chosen to be a binder to make magnetic recording medium. The recipe of Example 1 in the patent is as follows:
______________________________________ batch charge water 120 methylcellulose 0.6 sodium laurylsulfate 0.2 lauroyl peroxide 1.2 vinyl chloride 100 vinyl acetate 8 allyl 2-hydroxyethyl fiber 5 diallyldimethylammonium 3 chloride methanol 120 ______________________________________
Thus, in U.S. Pat. No. 5,064,730, a mixture of solvents such as water and methanol is also used. Once again, the usage level of catalysts and dispersants are unusually high compared with a typical suspension polymerization. Furthermore, two functional groups, namely hydroxyl group and quaternary ammonium salt are simultaneously incorporated into the polymer.
Approximately 80% of polyvinyl chloride resins are produced by suspension polymerization. The remaining 20% are produced by emulsion polymerization including the other methods such as solution polymerization and inversion polymerization. In spite of the latent commercial importance of doing so, hydroxyl groups have not been incorporated into polyvinyl chloride resins by suspension polymerization. Environmental concern alone may exclude the polymerization in hydrocarbon solvents. The presence of hydroxyl groups in the resins would allow greater versatility of the resins applications to the end products because of the polarity and reactivity of the hydroxyl groups.
The polyvinyl chloride particulates also need to have voids open to their surfaces. Voids which are sealed off at the surfaces are not useful, as they do not add to porosity. High porosity in the particulates permits faster removal of vinyl chloride monomer from the particulates at the stripping stage of the process, and a larger absorption of plasticizers, properties important in the subsequent commercial use of the resin. Up to the present, polyvinyl chloride resins have not been produced having high porosity and reactive hydroxyl groups available for postpolymerization reactions.